A radome is a housing for an antenna, radar or any type of seeker, and it is typically found on aircraft, missiles, submarines, moving vehicles or used in land or space based communications systems. A radome housing may include shapes such as a hemisphere, ogive, cone, cube, etc. A radome includes a frequency selective surface (FSS) which allows predetermined frequencies to pass through the housing, and it prevents undesired frequencies from penetrating the housing. When a FSS is used as a reflecting surface, it allows all frequency bands except one to pass, e.g., a bandstop reflector. The inverse of a bandstop is a bandpass. Frequency selective surfaces have been produced in much the same way as printed circuit boards. The main difference is that the FSS is usually printed on a three-dimensional doubly curved structure as in the case of a radome.
Presently, three-dimensional printed circuit boards and radomes are fabricated using a planar or substantially two-dimensional photomask. For example, a photoresist coating is applied to a three-dimensional metal surface, such as copper, which is disposed upon an underlying substrate. A substantially two-dimensional photomask of an element of a pattern is created using photolithography techniques. The element image of the two-dimensional photomask is transferred to a substantially planar portion of the photoresist coating using a standard incoherent ultraviolet (UV) light source. The process is repeated until all the element images have been formed in the photoresist coating. A photodeveloper removes the photoresist and exposes the image which allows acid etching of the copper surface to create the final pattern.
One possible method for transferring the element image to the substantially planar portion of the photoresist material is to use a contact imaging technique. The substantially two-dimensional photomask of an individual element is disposed in a contact imaging head. The illumination from the incoherent light source is directed into the contact imaging head, and the illumination forms an element image in a substantially planar portion of the photoresist material. The contact imaging head is repeatedly moved and stopped in each substantially planar portion of the photoresist material until the pattern is completely formed across the entire three-dimensional surface of the photoresist material.
The contact imaging technique described above is extremely slow, because the imaging head has to be physically moved and stopped in each substantially planar portion of the photoresist material to form an individual element of the pattern. Accordingly, there is a need for a conformal photomask which can image the entire surface of the three-dimensional printed circuit board or radome at one time,